It has happened to me a few times.
Once, while attending a conference, she was working in a booth for one of the exhibitors.
We recognized each other, smiled, but said nothing of it.
Obviously she did not work for that company, but was hired as a "booth bunny", to let her "attractiveness" draw more people.
That was a sideline of hers.
Another time, I was at a fast food place, and noticed a MP.
She noticed me also, but made efforts to not make it known.
Her (mother) was with her - so it was obvious that she did not want any confrontation.
So in this case - we just knowingly ignored each other's presence, out of respect.
On another occasion, at a restaurant, she was with her boyfriend.
She noticed me, and I could almost see the terror in her eyes - as if to plead with me not to approach her or say anything.
Of course I would never do that to anyone.
So, when he was not looking, she gave me a thankful (warm) smile, and we each went about our normal business.
Lastly - there have been quite a few occasions, where I am in public and an attractive woman locks eyes with me, and I am not sure that I even know her.
So then there's this confused trance that we exchange where we are both trying to figure out if we know each other, and if so.... from "where", Lol.
Or, perhaps I am just reading it wrong and it is a simple matter of attraction - who knows - but it is still interesting when it happens.
It is probably safest for both parties to not acknowledge each other - as you never know who's looking, or who is going to show up at any moment, or .....
Just respect each other's privacy and the fact that they have their own lives to live apart from whatever you and she have shared together in the past.
That's my .02 anyway.